1. Field of the Invention
The invention relates to the field of optical flow measurement or computation and, more particularly, to an optic flow sensor comprising elemental motion detectors, or the equivalent, wherein the detector outputs are fused to produce more robust optic flow measurement.
2. Description of the Related Art
As set forth in my earlier U.S. Pat. No. 6,020,953, the term xe2x80x9coptical flowxe2x80x9d generally refers to the motion of texture seen by an agent (such as an animal or a robot) as a result of relative motion between the agent and other objects in the environment. It is well known that animals, especially insects, use information from the optical flow for depth perception and to move about in an environment without colliding into obstacles. Robotics and machine vision researchers have borrowed from these ideas in biology to build machine vision systems that successfully use optical flow for depth perception and obstacle avoidance. These successes verify that optical flow can indeed be used for depth perception and obstacle avoidance in real systems. In a xe2x80x9cneuromorphicxe2x80x9d approach, computations are performed with analog or mixed-mode circuitry which exploit the physical dynamics inherent in VLSI circuitry and often mimic biological structures.
The aforementioned patent, the subject matter of which is hereby incorporated by reference, discloses a novel motion sensing algorithm, now referred to as the competitive feature tracker (CFT) algorithm, which is amenable to implementation in VLSI. Although this algorithm is able to measure optic flow when exposed to real-world textures and, in this regard, is able to measure optic flow when the texture contrast is made significantly lower than that found in nature, the algorithm can still make erroneous measurements especially as the texture contrast becomes lower.
In accordance with the invention, an optic flow sensor device is provided which significantly reduces errors inherent in prior art optic flow sensor devices and thus produces substantially more robust optic flow measurements. The invention can be used to fuse the outputs of elementary motion detectors of the competitive feature tracker (CFT) type disclosed in U.S. Pat. No. 6,020,953 as well as in connection with other types of prior art elementary motion detectors. Further, while the discussion of the invention which follows focuses on linear measurements, the invention can also be expanded to two dimensions.
In accordance with a first aspect of the invention, an optic flow sensor is provided which comprises: a plurality of photoreceptors, the photoreceptors generating a plurality of photoreceptor signals; a plurality of feature detector arrays, each said feature detector array receiving as input said photoreceptor signals, and each said feature detector array generating a plurality of feature signals; a plurality of optic flow measuring circuits or algorithms, each said optic flow measuring circuit or algorithm receiving as input one said plurality of feature signals, and each said optic flow measuring circuit or algorithm generating an optic flow measurement signal, whereby said plurality of optic flow measuring circuits or algorithms generates a plurality of optic flow measurement signals; and a fusion circuit or algorithm receiving as input said optic flow measurement signals, and generating a final optic flow signal from said optic flow measurement signals, whereby said final optic flow signal is a robust measurement of the optic flow as seen by said photoreceptors.
Preferably, each of said feature detector arrays generates a different feature signal from that generated by the others of said plurality of feature detector arrays.
Advantageously, at least one of said feature detector arrays comprises a plurality of edge detectors, and/or a plurality of second derivative detectors.
Each of said feature detector arrays preferably comprises a plurality of linear feature detectors although as indicated above the invention is applicable to two dimensions as well.
In one preferred embodiment, the fusion circuit or algorithm comprises a leaky integrator circuit or leaky integrator algorithm. In a further preferred embodiment, the fusion circuit or algorithm determines the recent history of the optic flow measurement signals and computes a mean or median based thereon to generate said final optic flow signal. In another preferred embodiment, the fusion circuit or algorithm determines a running average of the optical flow measurement signals to generate said final optic flow signal.
In accordance with a further aspect of the invention, an optic flow sensor is provided which comprises: a plurality of photoreceptors for generating a plurality of photoreceptors signals; a plurality of feature detector arrays, each of said feature detector arrays receiving said photoreceptor signals and each of said feature detector arrays generating a plurality of feature signals based on said photoreceptor signals received thereby, each of said feature detector arrays generating a different feature signal from that generated by the others of said plurality of feature detector arrays; a plurality of optic flow measuring means for receiving said feature signals, each said optic flow measuring means receiving as input one said plurality of feature signals and each said optic flow measuring means generating an optic flow measurement signal based on the variation over time of the respective feature signals received thereby, so that said plurality of optic flow measuring means generates a plurality of optic flow measurement signals; and a fusion means for receiving said optic flow measurement signals and for generating a final optic flow signal based on said optic flow measurement signals received thereby, said feature detector arrays being sufficiently different from one another that said optic flow measurement signals generated by said optic flow measuring means tend to have errors at different times.
As indicated above, the optic flow measuring means can comprise an optic flow measuring circuit or can comprise an optic flow measuring algorithm implemented by a computer.
As in the other aspect of the invention, at least one of said feature detector arrays advantageously comprises a plurality of edge detectors and/or a plurality of second derivative detectors.
Also as above, each of said feature detector arrays preferably comprises a plurality of linear feature detectors, with the proviso noted above.
As with the other aspect of the invention, there are a number of preferred embodiments of the fusion means, and, in this regard, the fusion means can comprise a leaky integrator circuit or leaky integrator algorithm, or a fusion circuit or algorithm which determines the recent history of the optic flow measurement signals and computes a mean or median based thereon to generate the final optic flow signal, or a fusion circuit or algorithm which determines a running average of the optical flow measurement signals to generate the final optic flow signal.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.